The strongest area of the roof is directly beneath the trim strip. This is where the two, or more, body panels are welded together. The 'trim strip' is simply an inexpensive way to conceal/finish the joint without a lot of expensive labor.

So, the most rigid mounting point is on the seam. Some makers like Mercedes Benz provide built in mounting points in this area. Also many European makers have a mark or indicators inside the door opening showing where to place the retaining clps.

Dan